Due to the improvement on manufacturing techniques and the reduction of cost, an image scanner becomes a popular peripheral equipment for a personal computer. Meanwhile, owing to the enhancement of the processing speed of the computer itself and the increasing requirement of users on the scanning quality, the resolution of an image scanner is getting higher and higher. Therefore, it is a trend for the manufacturers of image scanners to improve the resolution as possible as they can.
The resolution of an image scanner is generally expressed by X DPI×Y DPI wherein DPI means Dot Per Inch. the number Y indicates the count of scanning lines per inch, which is determined by the moving speed of a carriage driven by a stepping motor along the Y-axis. The number X represents the optical resolution which is determined by the amplifying rate of the lens device and the pixel number of the photoelectric converting device such as a charge coupled device (CCD). For X=600 as an example, it means that each inch of the scanned image in the X-direction, through the focusing and contraction of the lens device, is exposed by 600 pixels of the CCD.
In order to enhance the optical resolution of the image scanner, a software interpolation method or a CCD of a high pixel number can be optionally used. If a software interpolation method is used to simulate a high-level state, no additional cost for hardware equipment is needed but there is more or less distortion for the obtained image. If a CCD of a high pixel number is used, the cost of the resulting image scanner will multiply increase. Therefore, various researches are performed in order to develop an image scanner of higher resolution, better scanning quality, and lower cost.
The Taiwanese Patent Publication No. 313286 filed Jul. 29, 1996, and incorporated herein for reference, discloses a twice-scanning process by using a low-pixel-number CCD to scan a half of a document, and then moved along the X-direction to scan the other half of the document so as to double the resolution compared to the only once scanning operation by a CCD of the same pixel number. This process, however, still has disadvantages. First of all, the long distance movement of the light-program device including the CCD and the lens device across a half of the document along the X-direction has adversely effect on positioning precision. In addition, the switching device for driving the light-program device to move that long requires a complicated and bulky structure, and thus consumes a lot of power. Further, it takes time to move such a long distance.